Overvoltage protection circuits are used to protect electronic devices from transient voltages or “power surges.” Such a power surge may occur when power or signal lines supplying a device experience an increase in voltage above a safe or acceptable level. To give but one example, modems connected to telephone lines require protection from surges on both their power lines and the telephone lines. A conventional overvoltage protection circuit is shown in FIG. 1.
The overvoltage protection circuit of FIG. 1 is arranged to protect a load 16 from an overvoltage occurring across nodes 14, 12. The circuit comprises a zener diode 2 connected across the nodes 14, 12. The current-voltage characteristics of a typical zener diode are illustrated in FIG. 2, which shows the current ID through the zener diode with respect to the voltage VD across the zener diode. As shown, when the reverse-bias voltage applied to a zener diode reaches the breakdown voltage |VZ|, the current through the zener diode increases rapidly while the voltage across the diode remains substantially constant.
Hence, the zener diode 2 of FIG. 1 will conduct reverse-bias current IZ when the voltage across the diode approximates the breakdown voltage |VZ|. The breakdown voltage of the zener diode 2 may be chosen so that the diode conducts current when the voltage across nodes 14, 12 is at a particular threshold voltage.
Applicant has appreciated certain disadvantages in the overvoltage protection circuit of FIG. 1 and related circuits. In particular, although the circuit of FIG. 1 operates to substantially prevent the full surge voltage from appearing at the load 16, the load 16 is exposed to the threshold voltage (i.e., breakdown voltage) of the zener diode 2 during the voltage surge. In certain applications, the exposure of the load 16 to the threshold voltage for this time period may be harmful to the device. However, if a zener diode with a lower threshold voltage is used, it may be triggered by signal and low-level noise, rather than surges alone. In addition, the load 16 will be exposed to the full surge voltage for a brief period before the voltage across the load 16 is stabilized at the threshold voltage. Exposure to the full surge voltage, even for a brief period of time, may also be harmful to the device.
It should be appreciated that while the exemplary conventional overvoltage protection circuit of FIG. 1 uses a zener diode 2, other conventional overvoltage protection circuits may use another device (e.g., a SIDACtor® device) in place of the zener diode. Such circuits also exhibit the deficiency of not preventing exposure of the device to the full surge voltage for the entire duration of the surge.
A further complication is presented by the reactive nature of some loads. For example, to meet the impedance specifications for loads intended for telephone line connections, a modem or other line interface often will have the configuration of FIG. 3, wherein a capacitance C is connected in series between (for balance) a pair of transformer windings L1 and L2 through which a connection is made to the tip (T) and ring (R) conductors of a standard telephone circuit. The protected device 17 is connected to one or more opposing transformer windings L3. The net effect of the series L-C circuit is to aggravate the problem of protecting device 17 from large surges across the tip and ring lines. If a zener diode is used, it effectively short circuits the series arrangement of L1, L2 and C, forming an undamped resonant circuit that could drive a high amplitude from winding(s) L3 into device 17, repeatedly.
Accordingly, a need exists for an overvoltage protection circuit suitable for use in telephone circuits and which does not expose the protected device to voltages at or above its threshold level for a time sufficient to do permanent damage—e.g., the full duration of the surge. The threshold level may be sufficiently high so that the overvoltage protection circuit is not triggered into a protective mode by mere noise.